Thermal lamination involves the application of heat and pressure to two or more materials being laminated. These materials can be in sheet or web form. One goal when laminating is to apply sufficient heat and pressure to complete the lamination. Ideally, this goal should be accomplished without causing excessive shrinkage or stretch, and wrinkles, ripples, or curl.
When materials, such as graphic arts proofing materials, are laminated, specific defects can occur. Examples of these additional defects include waves, waffle, color density loss, and microbubbles. Waves are a type of tipple which angle diagonally along the length of the materials and have consequently been referred to as "Christmas Tree" waves. Waffle is another defect made up of numerous criss-crossing tipples giving a quilted appearance to the affected area. Color density loss is the undesirable fading and/or inconsistency of colors within the laminate.
Several approaches have been taken to generally address wrinkling and the other defects caused by the thermal lamination process. U.S. Pat. No. 4,861,409 describes a thermal lamination process which automatically adjusts the distance of contact of two films with the heat roll. This allows for a constant amount of heat to be transferred to the films even when the lamination linespeed varies. The constant amount of heat is noted to provide sufficient bond strength without overheating which can cause creasing and clouding of the films. This invention is noted by the inventors to be particularly useful just after start-up of the laminating process or just before stopping the laminating process. But, this approach has not satisfactorily solved the wrinkling problem, nor any of the other previously mentioned problems.
U.S. Pat. No. 4,946,640 describes an apparatus having a guide member surface and the application of a vacuum to the laminate materials. The vacuum creates a generally uniform drag force across the width of the laminate materials. This drag force is noted by inventors to eliminate wrinkles which form naturally at the guide member surface. However, this approach is relatively complex and expensive with the additional cost of including and maintaining the vacuum-providing hardware.
Graphic arts materials can be especially vulnerable to wave, waffle, and color density loss. The lamination of certain graphic arts materials commonly involves transferring four colors from four film-based color sheets (cyan, magenta, yellow, and black) to a paper base sheet. The first color sheet is laminated to the base sheet to create a proof, which is then exposed to create a first color image. The color sheet backing is then stripped off leaving at least a portion of the colored material from the color sheet behind on the base sheet. The remaining three colors can be transferred to the base sheet and exposed in the same manner. The four-color image created on the base sheet can instead be made on a transfer sheet, and then, can be ultimately transferred from the transfer sheet to a base sheet. Color sheets and transfer sheets commonly include a polyester film backing, wherein base sheets commonly include a paper backing.
Known apparatus and methods for laminating these materials do not satisfactorily thermally laminate the materials while minimizing wrinkling, curl, wave, waffle, and color density loss.